1. Field of the Invention
This invention relates to a liquid crystal panel. In particular, it relates to a liquid crystal panel utilizing a state in which a liquid crystal is vertically aligned when no voltage is applied.
2. Description of the Related Art
Liquid crystal displays have been developed rapidly and it seems they are substituting for substantial part of CRT (cathode ray tube) displays. From the top to the bottom in FIG. 5 illustrating a manufacturing method for a liquid crystal display panel that has been generally performed, first, a substrate 2 that has not an alignment control film coated thereon is prepared according to step S51, an alignment control film 6 composed of a polyimide, polyamic acid or the like is coated on the substrate 2 according to step S52, and then baking is performed according to step S53 followed by aligning treatment through rubbing. Thereafter, according to step S54, two such substrates are bonded together, and a liquid crystal 1 is introduced into the space according to step S55 to form a liquid crystal display panel.
For liquid crystal displays (LCD's) with an active matrix, liquid crystal panels according to the TN (twisted nematic) mode have been widely applied wherein liquid crystal materials having a positive dielectric constant anisotropy are aligned horizontally along the substrate surface and twisted at 90° between the two substrates that are facing each other. However, the TN mode entails, a problem of bad visual angle characteristics, and there have been various investigations conducted in order to improve them.
As a method substituting for this, there is the MVA (Multi-domain Vertical Alignment) system wherein a liquid crystal material having a negative dielectric constant anisotropy is vertically aligned when no voltage is applied, and the tilting direction of the liquid crystal at no voltage application is regulated by the uneven parts installed on the substrate surfaces and/or slits of an electrode or electrodes. This method has been successful in greatly improving the visual angle characteristics.
The explanations of liquid crystal panels according to the MVA system follow with examples in FIGS. 1A, 1B and FIG. 2. FIGS. 1A and 1B illustrate a model perspective view showing the alignment of a liquid crystal in a liquid crystal panel according to the MVA system, and FIG. 2 is a model plan view showing the alignment direction of a liquid crystal in a liquid crystal panel according to the MVA system.
In a liquid crystal panel according to the MVA system, a liquid crystal 1 having a negative dielectric constant anisotropy between two glass substrates is aligned vertically to the substrate surface as shown in FIG. 1 (A) at no voltage application. Pixel electrodes connected to TFT's (thin film transistors, not illustrated) are formed on a glass substrate 2, and a counter electrode is formed on the other glass substrate 3. Uneven parts 4 are formed alternately on the pixel electrodes and the counter electrodes, respectively.
When the TFT is off, that is, when no voltage is applied, the liquid crystal is aligned in a direction vertical to the substrate surface as shown in FIG. 1A, and when the TFT is on, that is, when voltage is applied, the liquid crystal is tilted towards the horizontal direction by the effect of the electric field, and the tilting direction of the liquid crystal 1 is regulated by the structure of the uneven parts. Accordingly, the liquid crystal molecules are aligned in plural directions in each pixel as shown in FIG. 1B. For example, molecules of the liquid crystal 1 are aligned, each, in directions A, B, C and D, when uneven parts 4 are formed as shown in FIG. 2.
Since it is not necessary for the MVA system to have an alignment control film in order to regulate the tilting direction of the liquid crystal, the aligning treatment process of an alignment control film represented by rubbing is not necessary, while it is almost always necessary in horizontal alignment systems such as the TN mode. This provides advantages in that the problems of static electricity and dusts in the rubbing process can be avoided, the cleaning steps after the aligning treatment can be eliminated, the problems of uneven alignment caused by fluctuation of the pretilt angle, etc. can be avoided, the facilities can be simplified owing to disuse of a printing machine for forming the alignment control film, baking furnace , etc., the processes can be simplified, the yield can be improved, and the production costs can be reduced. However, installation of the alignment control film is still needed in this MVA system.
Therefore, if the installation of the alignment control film can be avoided in the MVA system, there will be a lot of advantages including improved qualities of the liquid crystal, improved yield, simplified facilities, simplified processes, and low costs.
Furthermore, omission of the alignment control film is particularly advantageous in consideration of the present circumstances wherein superjumboization of the mother glass for substrates of liquid crystal panels has been developed rapidly, and it is becoming more and more difficult to have a printing machine for alignment control films corresponding to the size.
On the other hand, regarding technologies for enhancing the alignment properties of a liquid crystal, there are a liquid crystal existing as independent particles in a ionizing radiation-cured resin matrix (for example, see Japanese Unexamined-Patent Application Publication No. 5-113557, claims), a polymerizable monomer having an alkyl side chain being cured with a liquid crystal (for example, see Japanese Unexamined Patent Application Publication No. 6-265858, claims), a polymeric network structure-coated layer (for example, see Japanese Unexamined Patent Application Publication No. 6-289374, claims), a liquid crystal material in which a photopolymerizable acrylate having a liquid crystal skeleton structure is included (for example, see Japanese Unexamined Patent Application Publication No. 8-15707, claims), etc. However, it is believed that the question on omission of the alignment control film has been still unanswered.